Slumping Euro Dogged by Doubts

After its biggest rally in months, some old doubts about the euro are creeping back in.

Investors cheered the European Central Bank's proposal to buy the debt of troubled euro-zone members, announced on Sept. 6. But the plan already has hit its first potential snag: Spain, the region's fourth-largest economy, hasn't yet requested aid, a key condition for setting a bailout in motion.